CA1063680A - Non-fibrosing cardiac electrode - Google Patents

Abstract

NON-FIBROSING CARDIAC ELECTRODE
Abstract Non-fibrosing implantable electrode apparatus for muscle stimulation, such as cardiac stimulation, comprising electrode means formed from a non-thrombogenic conductive material and encapsulating means formed from a non-thrombogenic insulating material.

Description

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In the field of bio-medical devices, varisus types of implantable stimulators have come into usaye. One such stimulator is the cardiac pacerO ~hese devices gen- -erally employ one o~ two types of electrode apparatus --epicardial or endocardial -- for administering the stimu-lating pulses to the patient's heart. Myocardial leads have electrodes which are secured directly into the myocar dium, usually through the epicardial surface. Endocaraial ~::
leads are passed intxavenously to~the heart and the tip o~
electrode is lodged in the apex o the right ventricle o~.
the hear.tO
As the development of improved myocardial and -~
endocardial electrodes has progressed~ the parameters such .. : .
a~ physical size, configuration, mechanical and electrical ~roperties have all been considered and their effect upon ~ ~ ;
the safety~ long term reliability and corrosion resis~ance .. . . . .
~ the electrodes have been dealt with sufficiently so that their contr~bution to electrode performance is generally understood.

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In addition to the above characteristics, a further ~` and not hitherto fully understood characteristic known as the stimulation threshold has also been studied. The stimu--lation th~eshold is defined as the minimum level of stimula-tion (minimum pacing pulse energy) needed to elicit an s effective cardiac contraction, each time the stimulus is :., .
- applied. Xn studies of the changes o~ stimulation threshold ~ various implanted electrodes over a period o~ time subse-quent to implantation, it has been noted that the stimulation - D~
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threshold ~ypically increases steadily from an acute thres-hold at the time of implantation to a peak value and then decreases from the peak value to a chronic st~mulation threshold which then remains subs~antially constant. The complete process which causes this unusual time varying char-acteristic of the stimulation threshold of electrodes used for cardiac stimulation has never been fully understood~
Electrodes which are pxesently used for cardiac stImulation are manufactured from material that may be termed relatively thrombogenic and ibrotic~ That is, ~he blood's contact with the electrode and lead material initiates a sequence of events that ultimately produces thrombus or blood clot over the portion of the e~ectrode in contact wit~ the blood; T~e physiological trauma which occurs due to the incision when an endocardial electrode is ~mplanted, and at the stimulation site produce tissue thromboplastin -- a thrombogenic agent. The presence of thromboplast~n in the vicinity of the electrode causes the formation vf a thrombus or a fibrin mesh thereon due to the polymerization of fibrinoge~
i~ the blood.
It is known that most materials will initiate the clotting mechanism at a blood-material interface. The plati-num or platinum alloy conductor and silicone rubber encapsu-la~ion or insulation typical of the prior ar~ electrodes will initiate the clotting mechanism and a buildup of fibrin will occur on the surface initially. Subsequent to the initial buildup, fi~rinolysis or dissolving o~ the clot will occur~
Prior to the removal of the thrombus by flbrinolysis, :: :

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collagen will invade the site of the thrombus and ~orm a stable plug as has also been known to occur in rest~icted blood vessels, for example~ .
The presence of an object such as an electrode in the blood flow will cause perturbations in the blood's flow field~ If conditions of static or recirculating ~low are set up, clotting will occur. In areas of rapid or turbulent ~low such as in the heart, the chances for such stagnation are minimal. That this general process is also ap~licable to cardiac electrodes i5 verified by the observa-.. . .
tion that ~ibrosis on cardiac electrode leads generally occuxs primarily in the vicinity o~ the electrode and also ~n areas o~ restxicted ~low such as in the vicinity of the - portion of an endocardial electrode apparatus which passes - , .
through the cardia~ valve, and not along the remainder of the silicone rubber encapsulation which is used to enclose .
-. and insulate the l~ads in the prior art devices. This obser~
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: ~ vation is noteworthy because silicone rubber is also known .

. to be thro~bogenic to some degree and could, therefore, be : expected to produce a thrombus if the other necessary condi-tions axe present.

; ~ We have observed that the variations in stLmulation .
threshold during maturation of an implant are consistent . with ~he ab~ve pxocesses~ First, the stimulation threshold increases as the fibrin mesh forms, then decreases due to . ~ .
~ibrinolysis o~ the fibrin mesh and then stabilizes at a chronic stimulation thresholcl substantially in excess of the acute or initial stimula~ion threshold as the colla~en : - 3 -' ' ' ~tabilizes the plugO Since both the original fibrin mesh or the resultant collagen plug are non-excitable cardiac tissue, the apparent surface area o~ the electrode is increased, the current density applied to the excitable '' .tissue is.decreased and additional energy is required to ' apply the stimulation to the excitable cardiac tissue through the layer of non-excitable ~issue.
. . In the prior art, the development of the fi~rous ~ .layer has generally been believed to be necessary since it serves to prevent the dislodging of the electrode. -We~ha~
.. ' discovered, however, that it is not necessary to xely upon ~ -~,.. ,, . the ~ormation of a f~brous layer to maintain electrode p~sl~ion and that it is.desl~ible .to s~bs~-~ntially reduce ,: ~ the chronic threshold'of the ~ardiac electrode and to min;~
,: , ; .~iæe or eliminate the peak~l~ of the stimulation threshold .~ . , which.has occurred Ln the past as the stimulation threshold . . . . .
hanges with the time from its acute to its chronic level, he resultant electxode can be opérated with a lower'power ,-. drain on the stimulation pulse generator which thereby : .: . .
, ~ either reduces the amount of battery power~required and/or -- . increases the operational life.of the pacemaker without the , ' .necessity o~ improving the ~erformance o~ the battery or ;, power source.

' ' . , The present invention overcomes the disadvantages ;; , ., o~ the prior art by elimination of the buildup of D thrombus- ~
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like material on a stimulation electrode through the use o '~

. . non-thrombogenic materials both for the electrode itself ' .' ',. and also in the insulatiny ma,texials used in conjunction ,: . . ,: -.
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with the electrode. Implantable electrodes made from non-thrombogenic materials in accordance with our invention exhibit minimal fibrosis and will, thus, have low stimulation thresholds and, accordingly, re-duce the output power demands placed upon the pacemaker for effective stimulation, permitting it to achieve a longer operating life without any change in the capacit~ of the power sources used. An additional advantage which is also achieved by the use of non-thrombogenic, therefore non-fibrotic materials, is that the avoidance of the build-up of the fibrous layer around the electrode will permit it to be readily removed and replaced at the time of the periodic replacement of the pacing pulse generator.
A primary object of our invention is to provide an im-; plantable stimulation electrode which has a low acute stimulation threshold which does not appreciably increase upon maturation of the implant.
A further object of our invention is to provide a stimulation electrode which can be easily removed due to a lack of fibrous buildup on the electrode.
; An electrode constructed in accordance with our invention .:, ^ 20 is characterized by a non-thrombogenic conductive electrode in conjunc-tion with a non-thrombogenic encapsulating or insulating material.
Thus, in accordance with the present invention, there is provided a non-fibrosing low stimulation threshold electrode assembly - for permanent implantation in an animal body:
a) at least one electrode formed from conductive material ~, . . .
with at least the body tissue exposed surface thereof formed from a ~' non-thrombogenic material; and :. , .; b) a conductive lead connected to each of said electrodes;
and c) encapsulating means formed from a non-thrombogenic in-.~ '. .
~ - 5 -sula-ting material, said encapsulating means covering said conductive lead and the connection of said conductive lead to said electrode while leaving a portion of the outer surface of each electrode expos-ed for contact with body tissue to be stimulated.
The foregoing and other objects, advantages and char-acterizing features of an electrode of our invention will become clearly apparent from the ensuing detailed description of the illus-trative embodiments thereof, taken together with the accompanying drawings depicting the same wherein:

Figure 1 illustrates one form of a stimulation electrode according to our invention; and Figure 2 is a graph illustrating stimulation threshold characteristics as a function of time.
In the Drawings The embodiment shown in Pigure 1 is an electrode con-figuration of a bipolar endocaridal stimulation lead which is normal-ly inserted into the right ventricle of the heart through a vein.
The overall configuration of the cardiac electrode shown in Pigure 1 is similar in appearance to known prior art electrodes. The stimula-tion electrode is formed from an encapsulating or insulating material 1 which encloses a pair of coil spring conductors 2 and 3 which are ` connected in turn to a conductive ring electrode ~ and a conductive tip electrode 5. The free ends of coil spring conductors 2 and 3 are - connected to a suitable pulse generator Cnot shown) which applies an appropriate pulse excitation to the conductors. The construction and use o electrodes similar to the ones shown in Pigure 1, is illustrat-J.S~ ' ' ed in a~patent to W. M. Chardack, 3,348,548, entitled, "Implantable ~! Electrode with Stiffening Stylet".
~!, The material conventionall~ used for the encapsu-~, 30 lation or insulation 1 is a silicone rubber such as .. .. ..
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~ilicone rubber availabLe from Dow Corning Corporation under the trade ~ SilasticO Although this material is substantially inert and well tolerated by body tissue, it is relatively thrombogenic and fibroticO
The material most commonly used in the prior art for the conductive ring electrode and the conductive tip electrode is either pure platinum or a platinum alloy such as 90% platin~m and 10% ir~dium both of which axe, as in the cas~ of the silicone rubber insulator, relatively throm-bogenic and fibrotic.
In the stimulation electrode of our invention, the buildup of a layer of non-excitable tissue around the elec-trocle ~s minim~zed ox su~sta~tlia-ll~ el-~m-l,~a~d by ~he use ~f non ~hromboge~c m~teri~s ~or-~o~h ~he e~ectrode contact 4 -and S -and the-encapsulating ma erial ~ ~sed to insulate the conductQrs and to separate the electrodes. Although the buildup of the fibrous layer normally occurs in the vici~ity .:, of the electrode contacts, we have discovered that the pro- ;

~ision of a non-thrombogen;c electrode alone does not totally elimi~ate ~he problem of fibrous buildup, aIthoug-h it does - help to produce a low acute threshold during the period ~.- . .
! immediately subsequent to implantation. The apparent reason for fibrous tissue buildup is due to the eects of the , relatively thrombogenic and fibrotic encapsulating material :'"31, which causés a fibrous sheath to ~te created at the surface of the encapsulation materlal. This fibrous sheath then extends and "grows" over the conductor electrode even ~hough a non-thrombogenic electrode is utilized. Our data indicates that ` use of a non-thrombogenic electrode in conjunction wi~h . . .

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a rela~ively ~hrombogenic encapsulation such as silicone rubber normally results in a substantial increase of the stimulation threshold after the electrode has been implanted, although the peaking of the stimulation ~hreshold is avoided.
In our invention, we have used a non-thrombogenic conductor for the conductive ring electrode 4 and the conductive tip electrode 5. We have found that a very hard pyrolytic carbon coating can be applied to a platinum alloy or other suitable metal electrode to comple~ely cover all e~posed surfaces to produce a suitable stimulation electrode ;
having non-thrombogenic characteristics when used in combina-~ion with a non-thrombogenic enapsulation material. It is -a~so po~,ib3e~ko ~m the ~oli~ el~c-t~-ode ~rom pyrol~tic carbon deposited on a graphite subs~at-e-. -Pyrolytic carbo~
ha~ng suitable properties is manuactured and sold by General Atomic Compa~y of San Diego, Caliornia, u~der the trade ~r~ - - , ~Y~ LTI Pyrolite Carbon. A process for making pyrolytic carbon is shown in U.S. Patent 3,399,969 by Jack G. Bokros and Alan S. Schwartz and assig~ed to Gulf General Atomic Gorp~ration. ~ -; We have found that a non-thrombogenic encapsulating or insulating material 1 can be made by applying a polysi-loxane - polyurethane block copolymer material of the type manufactured by the Avco Corporation, Cincinnati,iOhio~ and ~ c~tk~
sold under the trade name ~WffYY~UY3~, over an extruded polyether urethane tubing. The manufacture of such a coating material is described in a United States Patent 3,562,352 .

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'" :',''~;' '' ' '' ' "''"' ' ''' '' '' ' " ' ~".''' ' ' ''' ''' ' ' by Emery Nyilas entitled, "Polysiloxane - Polyurethane Block Co-Polymers". In addition to having non-thrombogenic properties, Avcothane is a good insulator and has elasto-meric properties superior to the silicon rubber normally used for the encapsulating or insulating material. An example of an acceptable extrudable polyether urethane for use in the tubing is material sold under the trademark Estane 571~Pl manufactured by B.F. Goodrich Co.
Since the use of non-thrombogenic materials for both the electrode and the insulator avoids or minimizes the buildup of fibrous material around the tip of the elec-trode, it is desirable to provide our stimulation electrode -with a flange or tine or other lodging means 6 to facilitate lodging of the tip electrode in the trabeculae of the right ventricle of the heart.
`~! Figure 2 illustrates the change in the stimu-~ lation threshold, shown along the vertical axis, with time, .:
shown along the horizontal axis. The solid line 20 shows the peaking of the stimulation threshold typical or a con-ventional electrode in a short time and the eventual decrease in the stimulation threshold to a somewhat lower and sub-stantially constant chronic value. The dashed line 30 shows the gradual increase o the stimulation threshold of an .. . . .
electrode with a non-fibrosing material such as pyrolytic carbon conductive portion and a conventional encapsulating material. The peaking effect is eliminated and the stimu-... . . .

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lation threshold increases to a chronic value equivalent to that encountered with a conventional lead..
Although the invention has been descr~bed with : reference to certain specific embodiments, it is to be understood that the description is solely for the purpose of illustration and that many variations may be made by . j those skilled in the art without departing from the scope `
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of the invention as set forth in the claimsO ~or example;

~t is ~nti~cip-a~ed ~-hat the non ~ibrotic charac~eristics : .. ., . ~
... . of our stimulation electrodes will make them useful for stimulation o~ non-cardiac muscles and nerves in vario~s ` : application~ where it is desirable to have a relatively low .~. ; `.sti.mt~la~ion.~,hxeshold .t~at.i~ ~t: s~ c~-~o s.~ni~-icant . . .
:, . ... . - ., -.. ~ d~ a-~or~ upon maturation o:f ~1~ irnplc~t of the stimula~tL~n i: ,: . , ~ x~*e am~d ~n-a~plications w~ere it is aesired that the -,- ~ fibrous buildup on the electrode be minimized to permit , . . . . .. .
. relatively easy chronic removal o the stimulation électrode.
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Although we have disclosed electrodes which have ; -: . be.e~ .~o~.me.d c.~her ~rom aon~e~t~alial~ s over which a -.

~-non~th~o~boge~ic, hard p~ro3~ic ca~rbon coa~ing ha-s beQn ~ - . . .
appl.ied, or ~rom graphitic carbon substrates upon which p~rolytic carbon has ~een deposited, it .is also possible to ~.
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form the electrodes entirely from the pyrolytic carbon matexialO . :
Although we have shown the use of a non-thrombo- - ..
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genic - polysiloxane polyurethane block oopolymer insulating material, it is to be undexstood that there are other non- ..
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~ thrombogenic insulating materiais available. For example, ., ~ , .

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~ 3 silicone rubber with powdered tungsten incorporated therein is also a non-thrombogenic insulator. This Lnsulating material is shown in United States Pa~ent 3,829,903 to Wayne H. Stati and ~ack L. Bone entitled~ "Method of Inhibiting Blood Clot on Silicone Rubber Medical Devices~
and is assigned to Dow Corning Coxporation.
.In order to provide a redundant protective coat-ing, it has been found use~ul to coa~ an electrode made according to our invention in serum albumin to form a non-~ibrotic c~i~g over the non-fibro ic electrode~
It is also antlcipated that othex non-~hxo~bogenic materials may be used ~or the electrode anA insulation without depa~t.ulg ~rom the ~iri:t o~ our invention~

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A non-fibrosing low stimulation threshold electrode assembly for permanent implantation in an animal body:
a) at least one electrode formed from conductive material with at least the body tissue exposed surface thereof formed from a non-thrombogenic material; and b) a conductive lead connected to said electrode; and c) encapsulating means formed from a non-thrombogenic insulating material, said encapsulating means covering said conductive lead and the con-nection of said conductive lead to said electrode while leaving a portion of the outer surface of said electrode exposed for contact with body tissue to be stimulated.

2. An electrode assembly as claimed in claim 1 wherein the encap-sulating means is formed with a mechanical lodging means to hold the electrode assembly in place after it has been implanted in an animal body.

3. An electrode assembly as claimed in claim 1 wherein said elec-trode is formed from a conductive material coated with a non-thrombogenic hard pyrolytic carbon.

4. An electrode assembly as claimed in claim 1 wherein said encap-sulating means is formed from a non-thrombogenic polysiloxane - polyurethane block copolymer.

5. An electrode assembly as claimed in claim 2 wherein said elec-trode is formed from non-thrombogenic, hard pyrolytic carbon.